1. Field of the Invention
The invention relates generally to solid state integrated receivers that include a plurality of light sensitive regions, including a light detector, and to methods and apparatus for protecting the light sensitive regions (other than the detector) from being directly affected by scattered or stray light. More particularly, the invention relates to methods and apparatus for blocking scattered light through the use of a radiation sensitive polymer layer which is easily applied to a semiconductor wafer during the receiver fabrication process. The polymer layer is transformed into a light blocking material as a result of a lithographic process in which preselected portions of the layer are irradiated. Any light sensitive regions which are to be protected are covered by the blocking material (i.e., a material that will not transmit light) at the conclusion of the fabrication process.
In a broader context the invention contemplates the use of thin film as a light blocking layer which protects light sensitive regions (e.g., circuitry) located on a semiconductor substrate, where the film is comprised of a radiation sensitive polymer that is transformed into a light blocking material upon being irradiated.
According to the invention, the polymer must be easy to apply and must adhere to the semiconductor surface. It must be possible to pattern the thin film layer in a simple manner so as to permit its easy removal from any detector areas, areas associated with bonding pads, etc. Furthermore, the resulting thin film must have a high optical absorbance at a preselected wavelength (for example, 850 or 1300 nm), and must be able to withstand high temperature operations, such as the temperature ranges required to perform die bonding.
Related methods and apparatus for use in fiber optic links, Direct Optical Connectors (DOCs), fiber plate fabrication, etc., to for example, reduce crosstalk between a plurality of channels through which light energy may be transmitted, are also contemplated by the invention.
2. Description of the Related Art
Well known methods and apparatus exists for reducing the affects of stray or scattered light in a variety of applications. For example, Heine et al in U.S. Pat. No. 4,768,878, issued Sep. 6, 1988, teaches the use of a mechanical block (a diaphragm) for reducing the scattered light components in a scanning light ray used, in conjunction with an optoelectronic receiver, to detect submicrometer range defects in a surface under test.
Other mechanical techniques for reducing the affects of scattered light include the light trap taught in German Patent 26 43 361, issued Mar. 30, 1978, to Joachim et al; and a light baffle for absorbing stray light in a liquid crystal display to achieve improved optical contrast, as taught by Hedman, Jr. et al in U.S. Pat. No. 3,728,007, issued Apr. 17, 1973.
Techniques are also known for applying a background layer or coating in electro-optical display devices to improve visibility. For example, Fasano in U.S. Pat. No. 3,971,724, issued Jul. 27, 1976, describes techniques for applying a black thick film in such devices to serve as a visual background layer. In particular, the composition taught by Fasano is suitable for application as a printing medium and compatible for application by standard thick film techniques which will form a layer, that upon firing under the usual thick film temperatures and atmospheres, will yield a dielectric layer having a black, deep matte finish. The layer is easy to apply and has a substantive affinity to substrates and components commonly used to fabricate optoelectronic display devices.
Still other techniques are known which reduce scattered light in electro-optical devices. Bauer et al, in U.S. Pat. No. 4,264,147, issued Apr. 28, 1981, describes an indicating device having an electro-optical light valve unit arranged behind a plate containing fluorescent particles. The indicating device includes a contrast filter, disposed behind the plate and unit in the direction of viewing, and a reflector disposed behind the contrast filter. The contrast filter will pass light of the spectrum which excites the fluorescent particles of the fluorescent plate; but absorbs the emitted fluorescent light and majority of the other light so that a background is either dark or of a specific color.
None of the aforementioned mechanical, thick film or specific wavelength absorption methods are well suited for use in present day optoelectronic photoreceivers, high speed (gigabit range) optical links, DOCs, etc., for one or more of the following reasons: (1) exposure of the object being fabricated to high temperature operations such as die bonding; (2) the difficulty in applying, patterning and otherwise using the prior art techniques in the very small regions (for example, where circuitry is etched on a semiconductor substrate) on which many types of optoelectronic circuits are fabricated; (3) problems with the adherence characteristics of certain light blocking materials placed in contact with a semiconductor (e.g., GaAs); removal of light blocking material from light detecting regions and areas associated with bonding pads, etc. Not one of the known prior art techniques for protecting light sensitive regions from scattered light satisfactorily address all of these problems. Accordingly, it would be desirable if such techniques were available.
Photolithographic techniques, which will be seen hereinafter to be part of a solution to the aforestated problems, are known for making electro-optical components, such as semiconductor lasers, LEDs, detectors, and associated circuitry, as well as lenslet arrays, which are suitable for use in fabricating Direct Optical Connectors (DOCs), etc.. These techniques, which can, for example, help reduce interchannel crosstalk in a DOC by rendering regions between channels opaque, are described in Volume 24, pp 2520-2525 of Applied Optics (1985) and Volume 27, pp 476-479 of Applied Optics (1988).
These references teach exposing photosensitive glass to light in the area outside the region corresponding to the lenslets. However, the aforesaid techniques do not protect light sensitive regions per se (for example, light sensitive circuitry) from scattered light, nor do they involve the application of an easily applied thin layer of material for protecting preselected light sensitive regions, etc.
Patent application Ser. No. 07/542,275, filed Jun. 22, 1990, entitled "Electro-Optical Connectors"now U.S. Pat. No. 5,093,879, and assigned to the assignee of the present invention, teaches the use of an integrated detector and preamplifier which forms a receiver array, in DOCs. Such an integrated receiver can be realized utilizing GaAs MESFET technology. It will be seen hereinafter that this type of receiver (which contains a plurality of light sensitive regions including the detector), is an example of where the invention described herein finds significant utility.
The aforementioned patent application, hereby incorporated by reference, also teaches coating each fiber in an imaging fiber plate, with a thin absorbing layer (an EMA, extra-mural absorber) for the filling of the portion of the interstitial region between fibers with special light absorbing glass to help suppress optical crosstalk. Again, these techniques (like those in the referenced Applied Optics volumes) particularly address crosstalk related problems, and deal with a glass material that absorbs light; not an easily applied polymer, of the type to be described hereinafter, that realizes the benefits contemplated by the invention.
The aforementioned copending patent application also suggests that (1) waveguides used in Energy Transfer Fiber Plates (ETFPs) can be formed from polymers, in particular, photosensitive polymers such as those described patent application Ser. No. 07/495,241, now U.S. Pat. No. 5,054,572, filed Mar. 16, 1990, IBM Docket No. Y0989-086; and (2) that the interface of end plates used in Remote Optical Connectors (ROCs), can be coated with light absorbing surface layers to minimize stray light, etc. However, there is no suggestion in the incorporated reference that photosensitive polymers can be used to protect light sensitive regions per se from scattered light, nor are any techniques set forth (and indeed none are known in the prior art) for achieving such protection using thin films applied to a semiconductor, etc., while solving the problems set forth hereinbefore.
Accordingly, it would be desirable to be able to use thin films that result from the exposure of photosensitive polymers to light in a photolithographic process as a light blocking layer which protect light sensitive regions (e.g., circuitry) located on a semiconductor substrate.
More generally, it would be desirable to be able to use thin films that result from the exposure of radiation sensitive polymers (whether light sensitive, electron beam sensitive, etc.), in a lithographic process, as a light blocking layer which protect light sensitive regions located, for example, on a semiconductor substrate.
Furthermore, it would be desirable to be able to use a polymer that is easy to apply to the regions to be protected and which adheres well to a semiconductor surface.
Still further, it would be desirable to be able to easily pattern the thin film layer so as to facilitate its removal from any detector areas, areas associated with bonding pads, etc.
Further yet, it would be desirable if the resulting thin film has a high optical absorbance at preselected wavelengths, and is able to withstand high temperature operations, such as the temperature ranges required to perform die bonding.